Regarding damage to an unconscious character the PHB says:
Damage at 0 Hit Points. If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death.
My question is do the characters accumulate damage that ISN'T at this hp max value. So if a hit at 30 damage would be just enough to kill the unconscious character , what about one hit at 15 in one round, and another hit at 15 in the next round? Does this kill them because it adds up to their max, or does it not kill them because it isn't the lethal hit that the rulebook seems to describe?
As a side question, if the characters do accumulate damage past 0hp, they are essentially going into negative hp. If this is how it works, does a healing potion have to heal that negative damage, or does any healing begin at 0hp and work upwards?